Daf Yomi - Sukkah 25 - A Sukkah in Mourning

Rav Acha rules that a mourner is obligated in the mitzva of sukkah. The Gemora explains why this is a novel ruling for we might thought he would be exempt from the mitzva based on the ruling that one who is suffering is exempt from the mitzva of sukkah and a mourner is in anguish, the Gemora teaches us that he's nevertheless obligated.

The Rosh explains this Gemora by stating that a mourner wants to be in solitude and he would rather be in his house enabling him to think about the recent loss and this is causing him grief, the Gemora teaches that the exemption of sitting in a sukkah due to suffering is only when it comes by itself, however here he is bringing this anguish upon himself and it is incumbent on him to appease his mind, thereby obligating himself in the mitzva of sukkah.